In charged-particle-beam (CPB) microlithography (i.e., microlithography performed using a charged particle beam such as an electron beam or ion beam), as in optical microlithography (i.e., microlithography performed using visible or ultraviolet light), obtaining accurate alignment between the reticle and the substrate is extremely important. Current microlithography apparatus include sophisticated devices for determining reticle-substrate alignment. In a CPB microlithography apparatus as currently available, these alignments involve impinging a charged particle beam on a mark on the substrate or substrate stage and detecting charged particles (e.g., electrons) backscattered from the mark.
A conventional method and apparatus for detecting an alignment mark in CPB microlithography are shown in FIGS. 3(A)-3(C). FIG. 3(A) shows certain aspects of such an apparatus employing an electron beam EB; FIG. 3(B) depicts certain details of scanning an alignment mark on the substrate (wafer) using the electron beam EB; and FIG. 3(C) shows a typical backscattered-electron (BSE) signal waveform obtained by scanning the alignment mark in FIG. 3(B).
The apparatus of FIG. 3(A) includes an electron-beam source (electron gun) 1 that produces an electron beam EB propagating downstream of the electron gun 1. The electron beam EB passes through an electron-optical system OS and irradiates a silicon wafer or other suitable specimen 7. The wafer 7 is mounted on a wafer stage 5. The wafer stage 5 is movable to allow exposure of the entire wafer 7 and to allow loading and unloading of the wafer 7 for exposure.
The electron beam EB propagating toward the wafer 7 is deflected by a scanning deflector 3 located downstream of the electron-optical system OS. The scanning deflector 3 is connected to a deflector controller 4. The scanning deflector 3 is controlled by the controller 4, which thus causes the electron beam EB to be scanned over the wafer 7 in two dimensions.
Whenever the electron beam EB irradiates the wafer 7 in this manner, backscattered electrons are generated and propagate in an upstream direction from the upstream-facing surface of the wafer 7. The backscattered electrons are detected by a BSE detector 9. As the BSE detector 9 receives backscattered electrons, it produces a corresponding electrical “BSE signal” that is routed to a controller 10 via an interface 8. The interface 8 comprises an amplifier and an analog-to-digital (A/D) converter that convert the BSE signal from the BSE detector 9 into input data usable by the controller 10. The controller 10 comprises an arithmetic processor that calculates a position of the alignment mark 7a based on the input data. Thus, as the electron beam EB is scanned across the mark 7a on the wafer 7, changes in the BSE signal are monitored.
Before the electron beam EB is incident on the wafer 7, the transverse profile of the beam is shaped by passage of the beam through a beam-shaping aperture (not shown) situated within the electron-optical system OS. Passage of the beam through the beam-shaping aperture trims the periphery of the beam to a desired size and shape before the beam is scanned over the alignment mark 7a. An exemplary alignment mark 7a is shown in FIG. 3(B), in which elements of the alignment mark 7a are defined by corresponding channels 7b etched into the surface of the wafer 7. Ideally, whenever the electron beam EB scans the alignment mark 7a, the resulting BSE signal exhibits a distinctive waveform that corresponds to the elevational profile of the alignment mark 7a, as shown in FIG. 3(C). Due to the distinctiveness of the waveform formed by passing the beam over the mark 7a, it is possible to detect the position of the alignment mark 7a from the obtained BSE signal waveform.
As an alternative to defining elements of the alignment mark 7a by etched channels in the wafer surface, the elements of the alignment mark can be defined by forming corresponding regions of a patterned layer of a heavy metal (e.g., Ta or W) on the wafer surface. Regions of the layer of heavy metal exhibit a high level of electron backscattering relative to the wafer material.
In the apparatus of FIG. 3(A), whenever the specimen 7 is a silicon wafer having well-defined crystal properties, the BSE signal waveform that is obtained includes components derived from the alignment mark 7a and from the crystal properties of the specimen 7 itself. These signals are detected simultaneously, and thus both contribute substantially to the BSE signal waveform.
For example, if the electron beam incident on a crystalline silicon wafer 7 has an energy of approximately 100 keV, then changes in signal amplitude originating from the crystalline properties of the wafer material will be nearly equal to changes in the amplitude of the BSE signal from an alignment mark formed by channels in the wafer surface. The resulting lack of differentiation in the BSE signal produced by the alignment mark versus by the wafer surface causes a significant reduction in the accuracy with which the position of the alignment mark can be detected.